sys/net/gnrc: Flag esp_now as 6LN
sys/net/gnrc: Flag esp_now as 6LN

In gnrc_netif_init_6ln() the flag GNRC_NETIF_FLAGS_6LN is accidentally not set
for esp_now devices. This commit fixes this.

basilfx/RIOT,kaspar030/RIOT,basilfx/RIOT,jasonatran/RIOT,ant9000/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,smlng/RIOT,kYc0o/RIOT,authmillenon/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,yogo1212/RIOT,kYc0o/RIOT,authmillenon/RIOT,yogo1212/RIOT,basilfx/RIOT,kYc0o/RIOT,ant9000/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,miri64/RIOT,OlegHahm/RIOT,yogo1212/RIOT,OlegHahm/RIOT,miri64/RIOT,jasonatran/RIOT,OTAkeys/RIOT,smlng/RIOT,miri64/RIOT,ant9000/RIOT,miri64/RIOT,authmillenon/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,OTAkeys/RIOT,jasonatran/RIOT,jasonatran/RIOT,OTAkeys/RIOT,kaspar030/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,yogo1212/RIOT,kaspar030/RIOT,smlng/RIOT,ant9000/RIOT,authmillenon/RIOT,OTAkeys/RIOT,basilfx/RIOT,OlegHahm/RIOT,smlng/RIOT,authmillenon/RIOT,ant9000/RIOT,kaspar030/RIOT,basilfx/RIOT,smlng/RIOT,OlegHahm/RIOT,miri64/RIOT,yogo1212/RIOT,kaspar030/RIOT,yogo1212/RIOT